This invention relates to a process for alkylating an alkylatable isoparaffin hydrocarbon with olefin hydrocarbons. More particularly, this invention relates to a process for producing alkylated hydrocarbon product from C.sub.4 -C.sub.6 isoparaffin hydrocarbons and C.sub.3 -C.sub.5 olefin hydrocarbon reactants utilizing sulfuric acid alkylation catalyst under alkylation reaction conditions including short contact time of hydrocarbon and acid catalyst with substantially no backmixing of the reaction mixture within the reaction zone. This invention further relates to an alkylation process for producing high octane alkylated hydrocarbons having superior quality as components for gasoline motor fuels.
Processes for alkylation of isoparaffin hydrocarbons such as isobutane, isopentane, isohexane and the like with olefin hydrocarbons such as propylene, butylene, amylenes, and the like, are well-known and widely used commercial methods for producing gasoline boiling range hydrocarbons. The C.sub.5 -C.sub.10 carbon number range alkylated hydrocarbon products of such alkylation reactions are particularly useful as motor fuel blending stocks because of their high motor octane and research octane values. The selectivity of commercial alkylation processes for highly banched isomers of alkylated hydrocarbons determines the maximum product octane values obtainable. Consequently, process configurations and reaction conditions are being actively sought which favor formation of the highly branched alkylate isomers and which impede side reactions such as olefin polymerization and polymer cracking. The olefin polymers and their cracked products have substantially lower octane values than the preferred alkylate hydrocarbons of similar molecular weight.
Good contact of reactant isoparaffin and olefin hydrocarbons with alkylation catalysts under alkylation reaction conditions is critical to production of desirable high octane alkylate hydrocarbons. In alkylation reactions employing sulfuric acid as catalyst, the olefin reactants are substantially more soluble in the acid catalyst phase than are isoparaffin reactants. Since olefin hydrocarbons undergo polymerization reactions in the presence of the 88-98 percent sulfuric acid solutions commonly employed as alkylation catalysts, operating conditions must be selected to insure good contact of olefin with isoparaffin in the presence of sulfuric acid catalyst so that alkylation reactions will occur in preference to olefin polymerization reactions. In commercial liquid phase alkylation processes this contact of olefin and isoparaffin is obtained by employing isoparaffin in substantial stoichiometric excess to the olefin and subjecting the olefin-isoparaffin-sulfuric acid reaction mixtures to high shear mixing such that emulsions of hydrocarbon and acid are formed. Volume ratios of isoparaffin to olefin charge of from 2/1 to 20/1 are employed to insure the availability of isoparaffin for reaction with the olefin, with isoparaffin to olefin volume ratios of at least 4/1 being preferred.
Generally, it is preferred that the acid phase be maintained as the continuous phase in the reaction emulsions formed such that the hydrocarbon is present as small droplets suspended in the acid phase. Sulfuric acid concentrations of 40 volume percent and above in an alkylation reaction mixture may result in acid-continuous emulsions. Consequently acid concentrations in the range of 40-70 volume percent of an alkylation reaction emulsion are preferred. It has, however, been disclosed that hydrocarbon continuous emulsions of isoparaffin and olefin hydrocarbon with sulfuric acid may be employed as alkylation reaction mixtures, and acid concentrations as low as 10 volume percent in the reaction mixture have been successfully employed.
Reaction temperatures found acceptable in the prior art for sulfuric acid catalyzed alkylation of isoparaffin with olefin are in the range of about -20.degree.F to about 100.degree.F, with reaction temperatures in the range of about 40-60.degree.F being preferred. Such alkylation reactions are exothermic, therefore, the reaction mixture is commonly cooled in the reaction zone to maintain the desired reaction temperatures. Cooling by both direct and indirect heat exchange techniques is commercially practiced. One widely practiced technique is to recover the hydrocarbon phase of a reaction mixture and vaporize a portion of the unreacted isoparaffin under conditions of reduced pressure, thereby substantially reducing the temperature of the unvaporized portion of the hydrocarbon phase. This cold liquid hydrocarbon is then used to cool additional reaction mixture in the reaction zone by indirect heat exchange means.
Alkylation reactions of isoparaffins with olefins have been carried out in both the vapor phase and the liquid phase. In the present application, only those reactions carried out in the liquid phase are under consideration. Therefore, reaction pressures sufficient to maintain reactants in the liquid phase at reaction temperatures are required. Consequently, pressures of from about atmospheric to about 100 psig, or higher are commonly employed in such alkylation reactions. Pressures above those required to maintain reactants in the liquid phase have no noticeable affect upon the alkylation reaction.
Commercial processes for sulfuric acid catalyzed alkylation of isoparaffin with olefins employ back-mixed reactor vessels equipped with high shear mixing devices such as impellers, turbine mixers, etc. Such reaction vessels are sized to provide sufficient residence time for conversion of substantially all olefin within the reaction zone. The high shear mixing is provided to insure good contact of isoparaffin and olefin reactant in the presence of sulfuric acid catalyst. As olefins are substantially more soluble in sulfuric acid catalyst than isoparaffins, and since olefins tend to polymerize in the presence of sulfuric acid catalyst, high shear mixing of the reaction mixture to insure good contact of isoparaffin with olefin is critical to production of a high-octane alkylate product. Consequently, substantial amounts of power are consumed in providing the required good mixing of reactants. This degree of mixing of a back-mixed reactor results in an almost homogeneous reaction mixture, with olefin concentration about equal throughout the reactor volume. As it is desirable to effect essentially complete olefin conversion in the reaction zone for production of higher octane alkylated product, the olefin concentration is quite low throughout the reaction vessel.
Reaction emulsion effluent from such a back-mixed reactor discharges into a settling vessel wherein hydrocarbon phase comprising unreacted isoparaffin and alkylated hydrocarbon is separated from a sulfuric acid catalyst phase by gravity settling. The separated hydrocarbon phase is fractionated, in a fractionation zone, for recovery of an alkylate product fraction and an isoparaffin fraction. Additionally n-paraffin impurities of the same or lighter molecular weight as isoparaffin may be fractionated in the fractionation zone. The separated acid phase and isoparaffin fraction are recycled to the alkylation reactor for contact with additional isoparaffin and olefin reactants.
Liquid phase processes for alkylation of isoparaffins with olefins in the presence of acid alkylation catalysts and employing non-back mixed (or tubular) reactors have been proposed. For example, see U.S. Pat. Nos. 3,213,157; 3,169,153; 2,910,522; 3,000,994; and 3,456,033. These processes employ about the same reaction temperatures and pressures, ratios of reactants, residence times, etc. as are employed in commercial back-mixed processes. The advantages of the non-back mixed reactor processes include lower equipment cost and improved alkylate octane value over back-mixed reactor systems. The major disadvantage is the difficulty of maintaining a reaction emulsion of hydrocarbon and acid. Consequently good contact of reactant isoparaffin an olefin hydrocarbons in the presence of acid catalyst is difficult to maintain throughout the length of the non-back mixed reactors. Such non-back mixed reactors may comprise either vertical or horizontal tubular configurations. Generally, isoparaffin and olefin reactants are mixed with acid catalyst at the inlet of a tubular reactor and a reaction mixture comprising unreacted isoparaffin, alkylated hydrocarbon product and acid catalyst is withdrawn from the outlet of the tubular reactor into a settling tank. In the settling tank a hydrocarbon phase comprising isoparaffin and alkylate hydrocarbon is separated by gravity settling from an acid catalyst phase. The hydrocarbon phase from the settling tank is fractionated to recover alkylated hydrocarbon product and isoparaffin. The isoparaffin fraction and acid-catalyst phase is returned to the inlet of the tubular reactor for contact with additional isoparaffin and olefin reactants.
The prior art discloses that such alkylation processes employing non-backed mixed reactors are effective for improving alkylate quality and octane values when acid catalysts are used in which substantial amounts of isoparaffin hydrocarbons may be dissolved. Hydrogen fluoride, which will absorb about 2.7 weight percent isobutane at 80.degree.F, is particularly effective in such alkylation processes. On the other hand alkylation processes employing acid catalysts which do not absorb a substantial amount of isoparaffin reactant do not produce an improvement in alkylate quality and octane number. For example, non-backed mixed processes employing sulfuric acid, which only absorbs about 0.1 percent isobutane at 80.degree.F, are not as efficient and do not produce alkylate of improved quality and octane value, as compared to high-shear, back-mixed alkylation processes.